The input was done by removing the AM Stereo decoder chip, and substituting an op-amp in negative gain on an adapter board. (It was also reverse-phase, but I figured that was relatively unimportant.)

Output was realtively easy. I followed the signal path past the pre-amp to the tone/volume/balance/fader, and where it led back to the board was where the input coupling capacitor was located. I simply tapped into the 4 outputs after the fader control and ran them to RCA jacks using some 63 ohm resistors and cat5 network wire. (I found that the pre-amp would sometimes oscillate without the resistors, especially when changing inputs.) I'd guess signal level at that point was pretty close to 500mV, as the amplifiers treated it similar to an inexpensive aftermarket deck, in terms of gain settings.

I have about a dozen boards coming in next week designed specifically for this very purpose. I have been doing custom line drivers for OEM radios for about 5 years, from simple two channel to others with aux inputs. The boards themselves are comprised of all SMD components, and utilize a BUF rail splitter (cleanest sound) for the OPA driver. I also include a half-second soft mute turn on circuit for those that need it. Each board is two channel, and must be installed by Envision Electronics...unless you REALLY know what you're doing and don't want a warranty. They are $110 each, installed...a bargain considering you retain factory cosmetics and seriously decrease your chances for getting it ripped out of the dash one night.